The present invention is particularly but not exclusively concerned with providing a power sharing detector for use in a portable computer which requires battery charging to be done concurrently with computer usage.
FIG. 1 is a block diagram of a battery charging system for such an application. It shows the basic structure of a switch mode power supply and battery charger for a portable computer in block diagram form. Mains power is converted from its ac form at a supply Y8 into a dc form along line Y7 to be fed to a battery via a constant current charger Y2 and to an auxiliary output AO which is for connection to a computer.
If power is needed by the charger Y2 for charging a battery pack Y4 connected to the charger Y2 and at the same time power is required to be supplied via the auxiliary output AO to the computer, the total power needed must be supplied from the supply Y8 via the power supply converter components Y1,Y6,Y17 and Y5. Thus, the result is that the power supply converter components must be designed to match the maximum possible power demanded from it.
For example, if the computer utilises 25 W at the peak of its operation and the maximum power needed for battery charging at its fastest rate is 20 W, the power supply converter has to be designed for 45 W, assuming lossless power conversion. However, at times when charging is done without the computer being used, then the power supply converter is under utilised.
If a power sharing concept is used, the power supply converter can be designed for only 35 W but can still meet both power demands. For example, the charger can be designed to perform two rates of charging, one for the original rate, requiring 20 W, and another, slower rate at 10 W. In this way, during charging using the faster rate of 20 W, when the computer is suddenly turned on, this can be detected by a charge controller and the charger can thus be switched to the slower charging rate of 10 W, thereby releasing 10 W of extra power for computer usage. Thus, 10 W will be used for battery charging and 25 W will still be available for computer operation. One known way for detecting the on/off state of the computer to implement power sharing is to detect the state of a computer on/off switch. This method of detection has two drawbacks. Firstly, since the charger controller is required to read the state of the computer on/off switch, they have to be close together, thus possibly limiting the charger controller to be designed within the computer. Secondly, whenever the computer is turned on, the charging rate is halved no matter how much actual power is being demanded by the computer. For example, the computer may be only drawing 4 to 5 W of power which the power converter could easily handle with the faster rate of battery charging. This means that the power converter is not always utilised to its optimum capacity.
Another known method is to have a signal coming from a central processor of the computer to indicate computer operation. However, this means that a line has to be run from the central processor to the charger, again limiting the location of the charger controller to be near to the central processor unit. Also it has the same inefficiency problem as the previous method and presents an extra task for the central processor to perform.